1. Field of the Invention
The invention relates to a Schlieren type ultrasonic wave observer system, particularly to a Schlieren type ultrasonic wave optical observer system.
2. Description of the Prior Art
The Schlieren phenomenon was found by the U.K. inventor (Robert Hooke) first in 1665. Afterward the German scientist (August Joseph Ignaz Toepler) manufactured the first Schlieren photography apparatus in the world.
As for the technological principle of so-called Schlieren photography apparatus, because the light pierces through the transparent medium with uneven density, the advance direction of pierced light beam will be refracted due to the refractive index of transparent medium is changed with density. Thus, the Schlieren photography apparatus utilizes the perturbation of fluid on the light wave to present the geometrical shape and concentration at the place with uneven density by the optical observation. It is able to convert the change of flow field unable to be seen by ordinary naked eye into visible image. Therefore, it is able to take the picture for shape of invisible air flow (or water flow). It is also able to observe the wave characteristics or change of hot air conduction through different medium, such as liquid, gas or solid.
As shown in FIG. 1, the conventional optical system with Schlieren photography apparatus normally comprises an adjustable knife edge, a lamp type light source and a set of lens, in order to observe the image directly. However, due to too many optical components and the limitation of lens focus, the system will be bulky and unfavorable to be carried. The optical components are very expensive, and it is very difficult to be made. Thus, the price can not be born generally, and the bigger lens is often unable to be customized. Thus, the vision of generated image will be limited, and full ultrasonic wave image will be unable to be obtained in a time.
FIG. 2 shows the conventional space wave filter system (such as optical signal treatment or Fourier transformation system), which is so-called 4-fold (4f) focus system. After the laser is expanded by beam expander, it becomes to the parallel light. After it passes the object plane, the coordinate will be (x1, y1) at this moment. The light wave piercing through the object plane will be the object function f(x1, y1). When the light wave reaches the rear focus plane (spectrum plane or knife edge) through first lens 1, the spectrum of object function with coordinate (u, v) will be able to be obtained. When the light wave reaches second lens 2, the fully similar image is able to be obtained on the image plane of second lens 2, but its coordinate is reversed completely with coordinate (x2, y2). After the coordinate is reversed completely, the same image of original object will be able to be obtained. This system only has an optical field (about 5 cm in diameter). When the full ultrasonic wave sound field is observed, it is necessary to carry out two times of photographing, which will increase the difficulty of photographing.
In the prior art such as U.S. Pat. No. 4,681,437, although a set of optical Schlieren system was established and the technology for installing Schlieren apparatus was proposed, no advanced recommendations were described for the size and application of optical field of vision.
In U.S. Pat. No. 3,847,484, the laser was used as the light source of optical Schlieren system. Compared to other light sources, the laser source is more suitable to be applied in the optical Schlieren system, and it was not necessary to reduce the temperature by the cooling water. The size of optical field and its actual application still were not described.
In U.S. Pat. No. 5,515,158, the reflective focusing type Schlieren system with single lens was established. This Schlieren system employs the mirror reflection line to concentrate the light source. When the light source of this Schlieren system pierces through the field to the reflective grating, a sheet of reflective light can be produced. After the reflective light is returned, it pierces through the flow field to the first lens, and forms the image at its rear again. The design of control and the application of beam expansion field still were not described.
In U.S. Pat. No. 3,582,185, an optical Schlieren system was disclosed. It comprises control system to control the light source and a barrier system. It looks like chess where the arranged mirror surface lies on the light route. The lens had been installed on the light route. Although it could provide a shield system for the image on the light route, and could prevent or allow the passing of light, the optical imaging structure of Schlieren system and the beam expanding way of system still were not described.
In the imaging process of optical system, if a flat pattern is placed at the front focus plane of an ideal lens (Fourier transformation lens), a precise Fourier transformation will be obtained on the rear focus plane of lens, and its spectrum function can be obtained.
It is known from the Fourier spectral theory of electronics, if the spectrum of signal is filtered, the noise of signal can be removed after the signal is recovered. Thus, this Fourier spectral theory can be simulated on the rear focus plane of lens. When the optical grating with different shape and size is placed, the spectrum of pattern can be changed. After the spectrum of pattern is imaged by a second lens, the spectrum of pattern will be treated by the optical signal. The optical grating placed on the rear focus plane of lens is the so-called space filter.
Thus, summarized from the above-mentioned description, the drawbacks of previous art include:    1. The conventional optical Schlieren measurement system is bulky, and it is not portable, thus it is unfavorable for the development of commercialized product.    2. The optical field of vision is limited, thus it is unable to observe the full flow field at a time.    3. When the optical field is expanded, the bigger lens has to be procured. It will increase the manufacturing cost and the difficulty of customization. Thus it is unfavorable for general commercialized development.    4. It is unable to carry out the synchronous time sequence and it does not have the adjustable micro-control single chip core technology.
Therefore, in order to raise the measurement efficiency of optical Schlieren measurement system, carry out the observation of full flow field, and produce more effective ultrasonic wave measurement, it is necessary to develop innovative optical Schlieren measurement technology, so as to raise the efficiency of optical Schlieren measurement system and reduce the research and development time and manufacturing cost.